This invention generally relates to bearing materials and more particularly to a method of making a PTFE based tape suitable for impregnation into a porous metal matrix.
Dry bearing materials, i.e., those which will operate as a bearing without the benefit of a lubricant applied to the bearing surface, are well known and may generally be considered to be of three main types: 1) homogeneous materials which may be molded or pressed and machined, any surface of which can be the bearing surface; 2) non-homogeneous materials generally taking the form of a backing material and a bearing layer, in which the bearing layer is a dry bearing composition; and 3) non-homogeneous materials generally taking the form of a backing material and an impregnated interlayer, and having a relatively thin surface layer over the interlayer, which surface layer generally cannot be machined without seriously adversely affecting the bearing capability of the material because of exposure of the interlayer. The present invention may generally be considered to relate to a composite material of the third type referred to above, except that the surface layer may be of significant thickness such that it can be machined without seriously affecting bearing capability. The present invention may have other applications as well.
Numerous situations arise wherein a bearing material is required to provide good wear resistance and low friction under conditions of moderate load and temperature while operating in a substantially dry or non-lubricated environment. A variety of composite bearing materials and plastic materials which incorporate desirable bearing characteristics and are of good wear resistance under the foregoing operating conditions have heretofore been used or proposed for use as bearing materials. Of the various dry bearing materials known, those based on polytetrafluoroethylene (PTFE) have perhaps received the most widespread use for this purpose. While dry bearing materials incorporating PTFE have been found to provide satisfactory performance under many different bearing operating conditions, an inherent disadvantage of such materials, when they are of the third general type described above, is, as noted above, the difficulty encountered in the finish machining thereof into bearings of the desired configuration and size. It is desirable therefore to retain the advantages of the third general type of bearing material, while at the same time providing the product with a degree of machinability. Among such advantages of this third general type of bearing material are: 1) a good bond between the lining and the backing (the porous interlayer being metallurgically bonded to the backing, and the polymer being keyed into the porous interlayer --(This is particularly important in the case of PTFE which is difficult to bond to a backing. Composites of the second general type referred to above in which the lining is PTFE based tend to have a weak bond.); 2) dimensional stability (shared with the second general type referred to above when the dry bearing composition lining is thin); and 3) high thermal conductivity (shared with the second general type referred to above when the dry bearing composition lining is thin).
Dry bearing materials consisting of a metal backing and a porous metal interlayer impregnated with PTFE are well known. Generally, the PTFE is applied to the surface of the porous interlayer as a thick paste obtained by the coagulation of a dispersion of micron size PTFE particles in water. Sufficient water is retained in the paste to make it amenable to roll impregnation into the porous interlayer. Metallic or non-metallic fillers may be incorporated by mixing them in powder with the PTFE before coagulation or at some stage during the preparation of the paste.
A composite bearing material of the third general type referred to above is normally produced as continuous strip, starting with a coil of steel, (which may be plated), to the surface of which bronze powder is sintered to form a porous bronze layer. PTFE paste, with or without filler(s), is generally applied to the surface of the porous bronze with a spoon. The strip then passes under a roller which spreads the paste over the surface of the bronze as a reasonably uniform layer. It then passes through a rolling mill which forces the paste into the porous bronze, leaving little or no paste above the bronze. The strip is then heated to a temperature in excess of 327.degree. C. during which process the remaining water is driven off and the PTFE is sintered.
The method of applying the PTFE described above has several disadvantages. First, the paste is typically applied in discrete spoonfuls making it difficult to achieve a perfectly uniform distribution over the bronze surface. Second, the process is typically labor intensive, since the operator with a spoon is often solely occupied with spooning the paste on to the strip. It has proved somewhat difficult to mechanize this operation.
There is a third disadvantage which applies to certain forms of the product. In a common form of the product, a single filler, in the form of lead powder, is incorporated into the PTFE. During roll impregnation of the PTFE/lead paste into the porous bronze layer, only a very thin layer of paste, usually less than about 30 microns (1.2.times.10.sup.-3 inches) thick is typically left above the surface of the bronze. The thickness of this surface layer is substantially unchanged during sintering of the PTFE. The surface layer has very low wear resistance, and in service the surface of the composite wears rapidly until the bronze surface is exposed in spots in the area of rubbing. Wear rate then falls to a low value. As wear proceeds, the proportion of bronze exposed to the area of rubbing gradually increases. When the proportion reaches approximately 10%, wear rate begins to increase and bronze is exposed at an increasing rate. Wear rate increases rapidly and the useful dry bearing life of the composite is near an end.
In other forms of the product, more than one filler is incorporated into the PTFE. If the fillers are appropriately chosen, the surface layer may be made to possess a high degree of wear resistance. A low wear rate is not dependent on the exposure of the impregnated bronze surface in the area of rubbing. In such forms of the product, it is an advantage to have a thick layer of PTFE above the bronze, since the dry bearing life of the composite is thereby increased.
The third disadvantage referred to above of applying the PTFE as a paste by spoon to the bronze is that it has proved difficult by this process to leave a thick layer of PTFE above the bronze during impregnation. If the consistency of the paste is sufficiently soft for spooning and spreading evenly over the surface of the bronze, it is often so soft that some excess PTFE is often squeezed off the edges of the strip during impregnation and is not retained above the bronze as a thick surface layer. It has therefore proved difficult to achieve the increased dry bearing life theoretically made possible by the incorporation of appropriate fillers in the PTFE.
Development of a technique for creating a tape of PTFE incorporating controlled amounts of filler(s), suitable for roll impregnation into a porous metal matrix or interlayer, has been necessitated by the peculiar nature of PTFE. Although PTFE is classed by polymer chemists as a thermoplastic, it does not melt like other typical thermoplastics. At its transition temperature of 327.degree. C., it changes to a rubber-like state generally unsuitable for melt processing. Tape of the type suitable for impregnation into a porous matrix cannot therefore generally be produced by extrusion like other thermoplastic polymers, such as nylon based materials, hexafluoropropylene based materials, or the like. The common method of producing sintered PTFE tape is to press and sinter a cylindrical block of the polymer, with or without the incorporation of fillers, and to skive off a tape from the surface of the cylinder with a knife. Unsintered PTFE can also be produced, and is commonly used for sealing threaded joints. Also, the use of a conveyor and compressing roller system including a belt made of a filtering or permeable material (such as felt), and the application of a vacuum to both the belt and to a PTFE based composition (which may include fillers) thereon, to produce PTFE based sheeting of tape is known. However, these forms of PTFE tape are not believed to be totally suitable for roll impregnation into porous metal sinter, since in some instances the PTFE is sufficiently strong even above its transition temperature to compact the porous metal interlayer instead of continuously and fully impregnating into it, as is often desired. Successful impregnation may under certain circumstances be obtained, but may require the application of pressure at elevated temperature for times often impracticable for a reasonable roll impregnation strip process required to operate at reasonably economic speeds.
It is accordingly a principal object of the present invention to provide a method for making a tape which includes PTFE therein suitable for impregnation into a porous metal matrix.
Another object of the present invention is to provide an improved method of producing a composite bearing material.
Still another object of the present invention is to provide a method of making a composite bearing material having a surface layer of significant thickness which may be machined without seriously affecting bearing capability.